2. Gypsum dehydration products

The manufacture of plaster is based on a relatively simple principle: the total or partial elimination of water from the gypsum. In practice, however, this operation has complex implications, involving crystallization phenomena that are difficult to control.

When subjected to heat, gypsum leads to a series of partially hydrated or anhydrous products. At around 100 C, the α or β semi-hydrates are obtained (depending on whether they are produced under steam pressure or in the open air, respectively) according to the reaction : ${\text{CaSO}}_4\cdot 2{\text{H}}_2\text{O}!{\text{CaSO}}_4\cdot \frac{1}{2}{\text{H}}_2\text{O}+\frac{3}{2}{\text{H}}_2\text{O}$